The present invention relates to a color palette RAM and a D/A converter. In particular, it relates to a color palette RAM and a current output type D/A converter for graphics applications.
A schematic circuit diagram of a color palette RAM in the prior art is shown in FIG. 24. As illustrated in FIG. 24, the color palette RAM 10 in the prior art is provided with a RAM 11 for storing color data and an address register 12 that holds an address input via an address input terminal and outputs it to the RAM 11. The address register 12 holds an input address and outputs it to the RAM 11 at the rise of a clock pulse CLK. Then, at the following rise of the CLK pulse CLK, the RAM 11 outputs color data that corresponds to the address that has been output by the address register 12.
In addition, FIG. 25 presents a schematic circuit diagram of a current output type D/A converter for graphics applications in the prior art. It is to be noted that in the example in FIG. 25, the D/A converter has a resolution of 4 bits. As illustrated in the figure, 4-bit color data are input via a color data input terminal to the D [0:3] terminal of a data register 23, and a signal output from the OUT [0:3] terminal of the data register 23 is input to the D [0:3] terminal of a decoder 22. A decode signal output from the SELECT [0:14] terminal of the decoder 22 is input to the SELECT [0:14] terminal of a current conversion circuit 21, and the signal output from the AN_OUT terminal of the current conversion circuit 21 constitutes the D/A converter output signal. In addition, a clock pulse CLK is input via the CLK input terminal to the data register 23 and the decoder 22.
A circuit diagram of the current conversion circuit 21, which converts input color data to a current for output, is presented in FIG. 26. As illustrated in FIG. 26, a plurality of current output circuits COC, e.g. 15 current output circuits COC, each of which outputs a constant current, are provided in the current conversion circuit 21, and current output circuits COC are selected with a SELECT signal provided by the decoder 22 in a quantity that corresponds to the input color data, so that the total current output from the current output circuits COC selected by the SELECT signal is output from the AN_OUT terminal of the current conversion circuit 21 to constitute the output signal of the D/A converter.
In addition, a circuit diagram that represents an example of a current output circuit COC is shown in FIG. 27. As illustrated in FIG. 27, the current output circuit COC is provided with a current source 30 for outputting a constant current, which is constituted of a PMOS transistor 30a and a PMOS transistor 30b whereby a selection is made as to whether the output current from the current source 30 is to be output from the I_OUT terminal or discharged to the ground by using switching elements (a PMOS transistor 31 and a PMOS transistor 32) based upon the SELECT signal provided by the decoder 22.
The structure described above is adopted because, when control is implemented to operate/stop the current source 30 based upon the SELECT signal, a certain length of time is required before the output current from the current source 30 becomes stabilized after the current source 30 is switched from the stopped state to the operating state and, in order to operate the D/A converter at high speed, it is therefore necessary that the current source 30 output a constant current at all times.
Next, the operation of the D/A converter illustrated in FIG. 25 is explained in reference to FIG. 28. In FIG. 28, a timing chart corresponding to the circuit diagram in FIG. 25 is presented. As illustrated in FIG. 28, when color data "0000" are input via the color data input terminal, the data register 23 holds the color data "0000" and outputs them to the decoder 22 at the following rise of the CLK. Then, at the following rise of the CLK, the decoder 22 outputs a SELECT signal for selecting the current output circuits COC in the current conversion circuit 21 based upon the color data output by the data register 23.
Through this process, when the color data "0000" are input via the color data input terminal, the signal output from the SELECT [0:14] terminal of the decoder 22 is "0000h," with the result that no current output circuits COC in the current conversion circuit 21 shown in FIG. 26 are selected. This sets the level of the current output from the analog output terminal of the D/A converter to 0.
However, when color data "0001" are input via the color data input terminal, the decoder 22 outputs a signal "0001h," which corresponds to the color data "0001" from the SELECT [0:14] terminal, resulting in the current output circuit COC [1] in the current conversion circuit 21 being selected, to set the level of the current output from the analog output terminal to 1.
Likewise, when color data "0010" are input, the signal output from the SELECT [0:14] terminal is "0003h," resulting in two current output circuits COC, i.e., the current output circuit COC [1] and the current output circuit COC [2], being selected, to set the level of the current output from the analog output terminal to 2.
In addition, when color data "1000" are input, the signal output from the SELECT [0:14] terminal is "00FFh," resulting in eight current output circuits COC, i.e., the current output circuits COC [1] through [8], being selected, to set the level of the current output from the analog output terminal to 8.
Furthermore, when color data "1111" are input, the signal output from the SELECT [0:14] terminal is "7FFFh," resulting in all the current output circuits COC being selected, to set the level of the current output from the analog output terminal to 15.
It is to be noted that, since the operation of the current sources 30 in the unselected current output circuits COC do not stop, as explained earlier, the output currents from the current sources 30 at the unselected current output circuits COC are discharged to the ground.
As explained above, in the current output type D/A converter in the prior art, the data register 23, the decoder 22 and the current conversion circuit 21 are provided, with the data register 23 holding input color data to output it to the decoder 22 at the rise of the CLK pulse. Then, at the following rise of the CLK pulse, the decoder 22 outputs the SELECT signal to the current conversion circuit 21 in correspondence to the color data output from the data register 23, and the current conversion circuit 21, in turn, outputs a current based upon the SELECT signal provided by the decoder 22.
However, in the color palette RAM in the prior art structured as described above, since a clock pulse is supplied to the RAM 1 even when a single address is input continuously with the consequence that the output data from the color palette RAM do not change, operations such as precharge are performed continuously. Performing these operations continuously causes an increase in power consumption, which works against the need for reduced power consumption.
In addition, in the current output type D/A converter structured as described above, in which the currents are output from the I_OUT terminals of the current output circuits COC in the current conversion circuit 21 that have been selected by the decoder 22, the output currents from unselected current output circuits COC are discharged to the ground with currents flowing constantly from the current source 30 in all the current output circuits COC regardless of input color data. Thus, this operation also causes an increase in power consumption which works against the need for reduced power consumption.